The invention is basically concerned with protecting the surface of a metal body against the deposition of carbon on that surface, and against the detrimental effects that result from such carbon deposition. Protection of the components of a furnace employed in the thermal cracking of hydrocarbons such as ethane, propane, butane, naphtha, or gas oil, to form olefins, such as ethylene, propylene, or butenes, is a specific area of concern. The invention is directed at avoiding, or at least lessening, the formation of carbon deposits, commonly referred to as coke, on the furnace components, such as the wall of a reactor tube, during a thermal cracking process. Therefore, the invention is described in terms of this particular utility, although its wider application will be apparent.
At the heart of a thermal cracking process is the pyrolysis furnace. This furnace comprises a fire box through which runs an array of tubing. This array may be a set of straight tubing fed from a manifold, but frequently is a serpentine array of tubing. In either case, the array is composed of lengths of tubing and fittings that may total several hundred meters in length. The array of tubing is heated to a carefully monitored temperature by the fire box. A stream of feedstock is forced through the heated tubing under pressure and at a high velocity, and the product is quenched as it exits. For olefin production, the feedstock is frequently diluted with steam. The mixture is passed through the tubing array which is commonly operated at a temperature greater than 650.degree. C. During this passage, a carbonaceous residue is formed and deposits on the tube walls and fittings.
Initially, carbon residue appears in a fibrous form on the walls. It is thought this results from a catalytic action, primarily due to nickel and iron in the tube metal. The carbon fibers on the tube wall appear to form a mat by trapping pyrolitic coke particles formed in the gas stream. This leads to buildup of a dense, coke deposit on the walls of the tubing and fittings.
The problem of carbon deposits forming during the thermal cracking of hydrocarbons is one of long standing. It results in restricted flow of the gaseous stream of reaction material. It also reduces heat transfer through the tube wall to the gaseous stream. The temperature to which the tube is heated must then be raised to maintain a constant temperature in the stream flowing through the tube. This not only reduces process efficiency, but ultimately requires a temperature too high for equipment viability. Also, meeting safety requirements comes into question. This may be due to an embrittling, carburization reaction between carbon and the tube metal, or to a catastrophic, metal softening. A shutdown is therefore periodically necessary to remove the carbon formation, a process known as decoking.
Numerous solutions to the problem of coking have been proposed. One such solution involves producing metal alloys having special compositions. Another proposed solution involves coating the interior wall of the tubing with a silicon-containing coating, such as silica, silicon carbide, or silicon nitride. In still other proposals, the interior wall of the tubing is treated with a chromium and/or an aluminum compound. A practice of incorporating additives, such as organic sulfur and phosphorus compounds, in the feedstock stream in attempts to passivate the tube metal surfaces has also been used in commercial processes.
Despite this intensive effort, the industry still faces the problem created by carbon deposition on the high temperature, tube metals. It is then a basic purpose of the present invention to provide a method of avoiding formation of carbon deposits on such metal surface.
A further purpose is to provide an improved material to inhibit coke deposition on a metal surface.
Another purpose is to provide a coated component for a thermal cracking furnace that resists carbon deposition during a thermal cracking process.
A still further purpose is to provide a method of inhibiting the deposition of carbons on a furnace component during a thermal cracking process.
Still another purpose is to provide a coating on the exposed surface of a furnace component to inhibit coke deposition on the component during a thermal cracking process.